Mmark recently became able to create manual pages. This removed the
dependency on 'ronn' and just uses mmark (Go program).

Re-hookup Makefile.doc to generate the correct header mmark needs to
see and regenate them all.

Spot checking a few pages suggest they look good and actually better
than rendered with ronn, esp. lists in lists.

Fixes #2757
Signed-off-by: Miek Gieben <miek@miek.nl>